Re: [Intel-gfx] [PATCH] drm/i915: Reject excessive dotclocks early

[Jani Nikula <jani.nikula@linux.intel.com>]

On Tue, 27 Sep 2022, Ville Syrjala <ville.syrjala@linux.intel.com> wrote:
> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>
> Make sure modes with crazy big dotclocks are rejected early,
> so as to not cause problems for subsequent code via integer
> overflows and whatnot.
>
> These would eventually be rejected in intel_crtc_compute_pipe_mode()
> but that is now too late as we do the clock computations a bit
> earlier than that. And we don't want to just reorder the two since
> we still want to check the final computed dotclock against the
> hardware limit to make sure we didn't end up above the limit due
> to rounding/etc.
>
> Fixes: 0ff0e219d9b8 ("drm/i915: Compute clocks earlier")
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

Too bad we can't really simplify and remove the check from connector
mode valid hooks. Or we could but it gets tricky with the bigjoiner 2x
stuff.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>

> ---
>  drivers/gpu/drm/i915/display/intel_display.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
> index eb8eaeb19881..04e8fa953bc7 100644
> --- a/drivers/gpu/drm/i915/display/intel_display.c
> +++ b/drivers/gpu/drm/i915/display/intel_display.c
> @@ -8130,6 +8130,17 @@ static void intel_setup_outputs(struct drm_i915_private *dev_priv)
>  	drm_helper_move_panel_connectors_to_head(&dev_priv->drm);
>  }
>  
> +static int max_dotclock(struct drm_i915_private *i915)
> +{
> +	int max_dotclock = i915->max_dotclk_freq;
> +
> +	/* icl+ might use bigjoiner */
> +	if (DISPLAY_VER(i915) >= 11)
> +		max_dotclock *= 2;
> +
> +	return max_dotclock;
> +}
> +
>  static enum drm_mode_status
>  intel_mode_valid(struct drm_device *dev,
>  		 const struct drm_display_mode *mode)
> @@ -8167,6 +8178,13 @@ intel_mode_valid(struct drm_device *dev,
>  			   DRM_MODE_FLAG_CLKDIV2))
>  		return MODE_BAD;
>  
> +	/*
> +	 * Reject clearly excessive dotclocks early to
> +	 * avoid having to worry about huge integers later.
> +	 */
> +	if (mode->clock > max_dotclock(dev_priv))
> +		return MODE_CLOCK_HIGH;
> +
>  	/* Transcoder timing limits */
>  	if (DISPLAY_VER(dev_priv) >= 11) {
>  		hdisplay_max = 16384;

-- 
Jani Nikula, Intel Open Source Graphics Center